Zirconium dioxide is being used to an increasing extent in unstabilized or stabilized form as a ceramic material for articles which are subjected to high mechanical and/or thermal stresses. Stabilization of the desired ZrO.sub.2 phases can be effected by incorporation of other oxides, such as, for example, Y.sub.2 O.sub.3 or CeO.sub.2, CaO and/or MgO, in the ZrO.sub.2 lattice. In order for the green compact produced from the doped ZrO.sub.2, for example by compression or slip casting, to have the desired sinter properties and the sintered moldings to have the required good mechanical and thermal properties, it is necessary for the oxide metered in to be distributed as uniformly as possible in the ZrO.sub.2 lattice. For good processing by powder technology, the powder should be free-flowing, and the powder particles should consist of loose agglomerates to ensure good compressibility and good sinter properties.
ZrO.sub.2 powders can be produced from baddeleyite (impure ZrO.sub.2) or zircon sand (ZrO.sub.2.SiO.sub.2) by alkaline digestion with NaOH or Na.sub.2 CO.sub.2 at high temperatures. Depending on the proportions and temperature conditions, Na.sub.2 ZrO.sub.3 and Na.sub.2 SiO.sub.3 and/or Na.sub.2 ZrSiO.sub.5 are obtained. These compounds are hydrolyzed and the hydrated zirconium hydroxide obtained is dissolved again in sulfuric acid for further purification. Thereafter, either basic zirconium sulphate [Zr.sub.5 O.sub.8 (SO.sub.4).sub.2.H.sub.2 O] or basic zirconium hydroxide is precipitated with ammonia, these being finally calcined to give zirconium dioxide. These processes are expensive, and the powders obtained contain relatively large crystallites which have combined to form hard agglomerates. These powders are therefore difficult to process into high-density sintered articles.
A process having fewer steps and giving purer ZrO.sub.2 involves the reaction of zircon sand with chlorine in the presence of carbon. The zirconium tetrachloride formed here is reacted with water to give zirconyl chloride, which serves as a starting compound for the preparation of ZrO.sub.2 powders. Since zirconyl chloride (ZrOCl.sub.2.8H.sub.2 O) can be synthesized advantageously in large amounts, it is a starting material of commercial interest.
A frequently used process for the preparation of ZrO.sub.2 powders based on zirconyl chloride comprises reacting an aqueous solution of zirconyl chloride with an aqueous solution of ammonia or of an NH.sub.3 donor and calcining the basic zirconium hydroxide separated off by filtration. The disadvantage of this process is that the hydroxide precipitates obtained are difficult to filter and the calcined products are hard agglomerates.
Powders which can be more readily processed by powder technology are obtained if the zirconium hydroxide precipitated from aqueous zirconyl chloride solutions is partially dehydrated by azeotropic distillation prior to calcination. This process is modified in some cases by rendering the hydroxide chloride-free beforehand by adding nitric acid to the zirconyl chloride solution, redissolving in nitric acid the precipitate obtained on precipitation with ammonia and carrying out reprecipitation with ammonia. The zirconium hydroxide obtained is converted with citric acid into the corresponding organic complex and is dehydrated by azeotropic distillation and then calcined. These processes are labor-intensive and cause environmental pollution owing to the oxides of nitrogen.
A possible method for obtaining metal oxide powders having approximately uniform particle sizes in the submicron range consists in the hydrolysis of metal salts in aqueous solutions under severe conditions at elevated temperatures, as described by E. Matijevic in: "Monodispersed Metal (Hydrous) Oxides", Acc. Chem. Res. Vol. 14, 22-29 (1981). In this process, precipitation in an aqueous solution is carried out in such a specific and controlled manner that only a single batch of crystal seeds is formed and the further formation of solid take place through diffusion-controlled deposition on the crystal seeds. Since no further crystal seeds are formed, the originally produced crystal seeds grow into uniform, larger particles. These monodisperse particles can be produced only in very dilute solutions, and the hydrolysis conditions, such as, for example, salt concentration, type of anion, pH control, temperature, etc., must not fluctuate greatly. Monodisperse ZrO.sub.2 powders have not been produced by this method to date.
European Patent 0,251,538 describes a process in which an aqueous solution of zirconyl chloride is heated for a prolonged period at temperatures below the boiling point of water. The zirconium hydroxide formed during this procedure is separated off from the solution and calcined. Particles having a diameter of 50 to 200 nm are obtained. In this process, isolation of the hydroxide particles from the solution is very difficult, and the ZrO.sub.2 obtained is not stabilized. In order to stabilize it, the particles must be resuspended after calcination, laden with the hydroxide of the stabilizer by alkaline precipitation and calcined again after isolation from the solution. These additional process steps, during which the conditions must be maintained exactly, make the process even more difficult to carry out.
Earlier literature, for example the work cited in Gmelins Handbuch der anorganischen Chemie [Gmelins Handbook of Inorganic Chemistry], 8th edition, Volume 42 (Zirconium), pages 303 to 306, discloses that solid ZrOCl.sub.2.8H.sub.2 O can be converted into ZrO.sub.2 by vigorous heating with liberation of H.sub.2 O, HCl and possibly ZrCl.sub.4. This method has not been developed into an industrial process to date. The products obtained are not high quality powders. Furthermore, subsequent, satisfactory stabilization of the ZrO.sub.2 is not possible.